I want to connect my computer to my home theater system to listen to music through the stereo. What cable...?

Dont think you want a usb to aux cord since sound from a computer comes out a headphone jack. you need a headphone jack to rca audio cable like this one http://cgi.ebay.com/3-5MM-1-8-HEADPHONE-JACK-TO-M-RCA-AUDIO-STEREO-CABLE-6_W0QQitemZ290243300013QQihZ019QQcategoryZ32838QQssPageNameZWDVWQQrdZ1QQcmdZViewItem this is what I have and use. Not easy cords to come buy in a store I dont think. Maybe a radioshack or a store like that may have them. But there prolly not nearly as cheap as ebay

You should get a 1/8" stereo cable if you want analog ouput to the RCA jacks in your receiver. It will fit in the output jack on your sound card, or perhaps your computer has additional ports in front. The headphone jack will use a different voltage than you want to use for the home theater. I strung together two cables with an adapter, but ideally you'd have a long enough one to reach around the room to your receiver (or pre-amp). Ideally, you'd set the volume on the computer so that it equates to similar volume settings on your amplifier (eg, -25 dB sounds as loud as when you're watching cable, etc.). Alternatively, if you wanted a direct digital connection, you'd use an optical or coax to plug into your receiver's digital inputs. In this way, you can get multiple channels of sound. Lastly, many audiophiles recommend better sound cards than what comes on the typical motherboard.